The present invention relates to deposition of solder paste for surface mounted components on a printed wiring board (PWB) and, more particularly, to an aperture pattern for the deposition of solder paste through the opposite surface of a printed wiring board for surface mounted components.
A printed wiring board is a flat plate or base of insulating material containing a pattern of conducting material. The patterned conducting material form traces which electrically connect electronic components mounted on the surface of the printed wiring board to form circuits. The surface mounted components are any of the basic electronic parts used in forming a circuit such as resistors, capacitors, DIP, integrated circuits and the like.
The patterned conducting material in addition to forming connective traces also forms pads, conductive areas on the surface of the printing wiring board. Pads are provided on the printed wiring board so that connection can be made to the surface mounted components. The electronic components are then mounted to the appropriate pre-determined points on the conductive trace and pad pattern on the surface of the printed wiring board. Single layer printed wiring boards have all the conductors, the traces, pads and surface mounted components on one side of the board.
In one method of mounting the electronic components on the surface of the printed wiring board, the pads can have through-holes formed from the pad through the printed wiring board. Each through-hole is coated with a conductive material along the walls of the hole. The electronic components will have connection pins which are inserted in the through-holes on the pads and then reflow soldered into position on the pads with their connective traces to the rest of the circuit.
Alternately, electronic components can be mounted on the surface of the printed wiring board using solder paste deposited through a stencil in a desired pattern on the printed wiring board.
The solder paste is deposited by a screen printing process using the stencil. A typical stencil will be fabricated from stainless steel foil having a thickness of the order of 0.005 or 0.006 inches (5 or 6 mils). Apertures are formed in the stencil at positions and shapes corresponding to the required component mounting pads on the printed wiring board. The stencil is precisely aligned in physical contact with the upper surface of the printed wiring board and held firmly together with the printed wiring board.
Solder paste is applied to the upper surface of the stencil. A squeegee is then drawn through the solder paste and over the entire stencil and the solder paste is pushed into each of the apertures of the stencil and forced through the apertures onto the printed wiring board to form the solder component mounting pads on the upper surface of the printed wiring board.
The surface mounted component itself will have contact pads or contact points. The electronic components are subsequently placed onto the printed wiring board with their contact pads mating with printed wiring board pad patterns onto which the solder paste has been precisely stenciled. The printed wiring board with electronic components thereon, then passes through an oven where the solder is melted or reflowed, thereby fusing the electronic component contact pads with the mating printed wiring board pads.
Although use of surface mounted components generally offers high circuit densities, such components generally have significantly less mechanical strength in terms of their physical connection to the printed wiring board than components mounted by means of plated through-holes.
An additional problem with components mounted on the surface of a printed wiring board is the heat sink for a component is typically mounted on the component between the component and the printed wiring board. This heat sink position provides a poor thermal path for dissipation of heat from the heat sink.
A further problem for components mounted on the surface of a printed wiring board is providing a low inductance electrical path to ground within the printed wiring board.
It is an object of this invention to provide a method and apparatus for deposition of solder paste for securely mounting electronic components on a printed wiring board.
It is another object of this invention to provide a method and apparatus for deposition of solder paste from the opposite surface of a printed wiring board for securely mounting electronic components on a surface of the printed wiring board.
It is yet another object of this invention to provide a thermal path for dissipation of heat from a heat sink attached to a surface mounted component from the opposite surface of a printed wiring board.
It is yet another object of this invention to provide a low inductance electrical path to ground within the printed wiring board for the component mounted on the surface of the printed wiring board.
According to the present invention, a method and apparatus for depositing solder paste on a printed wiring board has a stencil with a pattern of multiple apertures. The pattern aligns with a through-hole in the printed wiring board when the stencil is in physical contact with the printed wiring board. A squeegee arrangement applies solder paste through the multiple apertures in the stencil through the through-hole of the printed wiring board to the lower surface of the printed wiring board for mounting electronic components to the lower surface of the printed wiring board.
The circular pattern of the multiple apertures aligns with the circular through-hole. The identical triangular shape and symmetrical pattern of the apertures forces the solder paste against the wall of the through-hole.
The resulting annular ring solder pattern in the through-hole from the opposite surface of the printed wiring board securely mounts the component to the printed wiring board, provides a low inductance electrical path to ground in the printed wiring board, and provides a superior thermal path for the heat sink of a surface mounted component.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.